Blades, in particular stator vanes, in a gas turbine engine, in particular in an axial-flow gas generator turbine, are subjected to mechanical and thermal loads during operation of the turbine. The thermal and mechanical loads are caused by hot gas flow heating up the vanes and applying gas forces to the vanes. In particular the first nozzle guide vanes immediately downstream of a combustor of the gas generator experience hot gas temperatures.
High demands are made on design and construction of vanes having sufficient mechanical integrity in order to withstand applied loads during operation.
Further, the integrity of the vanes also depends on the life endurance of the vanes. In particular, when a vane is subjected to a high temperature in combination with a high strain for a long period, creeping of the vane can occur resulting in cracks in the vane material and finally in mechanical failure.
The strength of the vane material is dependent on stresses applied during operation, operation temperature and operation time. In order to improve the mechanical integrity and life endurance of the vane it is a common remedy to cool down the vane material.
The vane is provided with internal cooling passages through which cooling air is flowing. The cooling air is extracted from a compressor of the gas generator which represents a significant efficiency and power output penalty.
A guide vane assembly of the gas generator turbine is comprised by a plurality of guide vane sections attached to one another. Each guide vane assembly comprises the vane and a hub portion and a shroud portion. Each hub portion of one guide vane section is abutting the hub portion of the adjacent guide vane section thereby forming a hub of the guide vane assembly. Each shroud portion of one guide vane section is abutting the shroud portion of the adjacent guide vane section thereby forming a casing of the guide vane assembly.
The partition of the guide vane assembly into guide vane sections is uniform such that each guide vane section is identical in its geometry and dimensions. Therefore, each guide vane section can be manufactured similarly. It is common to manufacture the guide vane sections by casting.
However, for cooling purposes the guide vanes of the guide vane assembly are provided with internal cooling passages. Since the geometrical dimensions of the guide vanes are small, it is difficult to manufacture the internal cooling passages within the interior of the guide vane material with respect to accuracy and reasonable manufacturing cost.
Given that the gas temperature experienced by the vanes in a modern gas turbine can reach or even exceed 80% of the melting temperature of the available nickel alloy materials, current technologies which rely upon casting internal cooling passages have been refined to a very high level. A principal barrier is the practical accuracy with which internally cast cooling features can be manufactured, especially in alloys with very advanced microstructure such as directionally solidified and single crystal materials. This tends both to reduce the efficiency of the cooling and result in larger passages which waste air to the detriment of the machine performance.
Furthermore, inaccuracies in casting mean that the cooling air distribution is usually far from even around the gas turbine, meaning that the design of the nozzle guide vane has to be set out for the worst case, and results in wasted air for almost all other nozzle guide vanes. This is particularly acute for small gas turbine engines, where casting tolerances are a much larger fraction of the part and passage size. This also means that the average wall thickness has to be greater than desirable to avoid weakness in the worst case. This results in greater thermal resistance and thus again reduces cooling efficiency.
It is an objective of the invention to provide an inlet guide vane arrangement for a gas turbine engine, wherein the guide vane duct element has a high cooling efficiency and nevertheless can be manufactured easily with high accuracy.